Southern Germany roughly corresponds to the area of Germany south of the Speyer line, below which Upper German
dialects are spoken (shown in green)
Southern Germany primarily contrasts with Northern Germany.
The term mostly corresponds to those territories of modern Germany which did not form part of the North German Confederation in the nineteenth century.
Between Northern and Southern Germany is the loosely defined area known as Central Germany (Mitteldeutschland), roughly corresponding to the areal of Central German dialects (Franconia, Thuringia, Saxony).
The boundary between the spheres of political influence of Prussia (Northern Germany) and Austria (Southern Germany) within the German Confederation (1815–1866) was known as the "Main line" (Mainlinie, after the River Main), Frankfurt am Main being the seat of the federal assembly. The "Main line" did not follow the course of the River Main upstream of Frankfurt, however, it corresponded rather, to the northern border of the Kingdom of Bavaria.
Linguistically, Southern Germany corresponds to the Upper German dialects. Southern Germany is culturally and linguistically more similar to German-speaking Switzerland, Austria, and German-speaking South Tyrol than to Central and Northern Germany. A jocular term referring to a cultural boundary defining Bavarian culture is Weißwurstäquator, i.e. the "equator" dividing Northern Germany from the homeland of the Weißwurst sausage.